Recovery of oil from oil shale involves heating the oil shale rock to sufficient temperature such that kerogen, the organic material contained in oil shale, converts to gases, liquids and residue. The residue, which may be a form of coke, is left deposited on the mineral matter of the oil shale. In the prior art the principal objectives of such processes are to (a) efficiently get the heat into the oil shale rock and (b) separate the desirable products from the spent oil shale.
The process of heating oil shale to conversion temperatures is generally called retorting, a word of European origin that describes a process for distillation or destructive distillation. The retorting process may be conducted in-situ by supplying heat to a bed of ore largely undisturbed. The retorting process may also be conducted by modified in-situ, by first preparing the bed of ore prior to heating. Retorting may also be conducted in surface vessels to which mined ore is introduced.
In the prior art, the methods for recovering oil values have exhibited various problems that adversely affect the efficiency or reliability, which in turn, adversely affect the economics. For example, vertical, solids down-flow, product up-flow processes have proven to be reliable, but yield a relatively low grade of product because the product is required to exit the reactor overhead, forcing the product to remain in the reactor longer than desirable. Vertically-inclined retorts that provide a means for removal of products from the bottom (product down-flow), and counter-currently pump the ore from the bottom by means of a ‘rock pump’ (solids up-flow) have proved to suffer from poor mechanical reliability.
Horizontally-inclined vessels, patterned after rotary kilns, suffer from slow heat transfer and result in much larger vessels than vertically-oriented vessels and are expensive to fabricate. True in-situ methods, while lower in capital costs, are not as controlled and result in uncertainty about legacy environmental issues such as ground water contamination. Modified in-situ methods experience better control than true in-situ, but might result in poorer control than surface processes. Modified in-situ processes are also likely to suffer from lower production efficiency compared to surface processes.
In historic retorting processes the production of CO2 from decomposition of carbonate minerals was not considered an environmental issue. However, it was considered an energy consumption issue in that decarbonation is an endothermic reaction and consumes up to 8% of the energy value available from the ore. Various proposals have been made to limit the amount carbonate decomposition that gives rise to these endothermic reactions, the most common being maintaining a temperature below which decomposition occurs. However, a lower final temperature results in a lower rate of production, and possibly lower product yields.
Tar sands are also processed to recover and convert the organic materials (bitumen) to marketable products. In typical practice tar sands are mined, mixed with water, and the bitumen (tar) is separated from the sand by flotation. In yet other practice pipes are laid, or drill-holes are made into a bed of the resource and steam is injected to raise the temperature of the bed. Viscosity of the bitumen is reduced, which then drains to a second lower pipe where it is withdrawn. While practiced in rich, unconsolidated ores such as those found in Alberta, Canada, not all resources in the United States or Canada are rich or unconsolidated. Some are consolidated, with low permeability and sufficiently lean (lower grade) that only small amounts of bitumen are recovered by water extraction or will drain in a steam stimulation process. In principal, tar sands can also be retorted.
Thus, there remains a need for a process that is efficient, reliable, environmentally friendly and cost-effective for both oil shale and tar sand resources.